浅埋大断面公路隧道渐进破坏规律与安全控制

围绕浅埋大断面公路隧道渐近破坏过程与机制，以大永高速公路甸头隧道下穿大西二级公路工程为背景，开展了室内相似模型试验与现场监测分析。针对不同围岩级别条件（Ⅳ₃，Ⅴ₁和Ⅴ₂），分析了隧道毛洞开挖过程中围岩应力和位移变化规律，提出了基于变形差率的隧道施工安全控制指标，隧道施工现场监测验证了该指标的科学性，保障了隧道施工安全。研究结果表明：①隧道的破坏首先发生在拱顶位置，随着隧道的不同分部的开挖，破坏区向拱肩和地表扩展；围岩级别为Ⅳ₃时，隧道开挖后围岩形成一定厚度的塌落拱，塌落拱高度约为0.67M（M为隧道最大跨度）；围岩级别为Ⅴ₁和Ⅴ₂时，隧道开挖将引起围岩坍塌，形成塌方等较严重事故。②隧道开挖过程中，在开挖卸荷作用下，隧道产生不同程度的收敛与沉降，Ⅳ₃比Ⅴ₁，Ⅴ₁比Ⅴ₂，Ⅳ₃比Ⅴ₂最大地表变形分别减少了63.0%、20.0%和70.4%；隧道开挖应力变化方面，Ⅴ₁比Ⅳ₃，Ⅴ₂比Ⅴ₁，Ⅴ₂比Ⅳ₃最大应力变化量分别减少了43.5%、23.0%和56.5%，且Ⅳ₃，Ⅴ₁和Ⅴ₂围岩级别下隧道开挖过程应力和变形影响范围逐个增大。③采用模型试验手段，通过计算典型测点沉降差与测点距离的比值——变形差率，分别探讨Ⅴ₂，Ⅴ₁和Ⅳ₃围岩级别的浅埋隧道施工安全控制标准。隧道施工现场监测结果表明，对于Ⅴ₁级围岩浅埋隧道，当隧道地表横向和纵向变形差率均小于10 mm·m^-1，可防止公路地表裂缝的产生。研究成果对于浅埋大断面公路隧道施工与安全控制具有重要意义。

Progressive Failure Law and Control Criterion for Safe Construction of Shallow Buried Highway Tunnel with Different Grades of Surrounding Rock

Based on the progressive failure process and mechanism of shallow-buried tunnels with large sections, model tests and in-situ monitoring were carried out; the Diantou Tunnel was used as the engineering background of this study. The stress and surface settlement of the surrounding rock during tunnel excavation were analyzed according to the different grades (Ⅳ₃, Ⅴ₁, and Ⅴ₂) of the surrounding rock. Consequently, a safety control index of tunnel construction based on differential settlement is proposed. The results of on site monitoring verify the scientific nature of the index, ensuring the safety of the tunnel construction. Conclusions can be drawn as follows:①The damage of the shallow buried tunnel begins from the arch crown; with the excavation of the different segments of the tunnel, the damage zone extends to the arch shoulder and the ground surface; when the surrounding rock grade is Ⅳ₃, the surrounding rock has a stable collapse arch. The height of the collapsed arch is approximately 0.67M (M is the maximum span of the tunnel); when the surrounding rock grades are Ⅴ₁ or Ⅴ₂, the tunnel excavation causes the surrounding rock to collapse, which can cause serious accidents such as landslides. ②During tunnel excavation, the tunnel has different degrees of convergence and settlement under the excavation unloading; for the maximum surface settlement, Ⅳ₃ is 63.0% less than Ⅴ₁, Ⅴ₁ is 20.0% less than Ⅴ₂, and Ⅳ₃ is 70.4% less than Ⅴ₂; for the maximum stress variation, Ⅴ₁ is 43.5% less than Ⅳ₃, Ⅴ₂ is 23.0% less than Ⅴ₁, and Ⅴ₂ is 56.5% less than Ⅳ₃; the influence range of stress and deformation under the grades of Ⅳ₃, Ⅴ₁, and Ⅴ₂ in the surrounding rock increases. ③Using the model test method, the ratio of the differential settlement/the measuring point distance was calculated; Then, the control criterion for the construction of shallow buried tunnels of the above three grades were discussed. As for the Ⅴ₁ surrounding rock grade, the monitoring results show that when the horizontal and vertical deformation differences of the tunnel are less than 10 mm·m^-1, surface cracks of the road can be prevented. Therefore, the research results are of great significance for the construction and safety control of shallow buried highway tunnels with large sections.